Air Brakes: Understanding Bus Safety Mechanisms

Air brakes are a critical component of diesel vehicles such as buses and trucks. They are used to enhance vehicle safety and are considered more reliable than hydraulic brakes, which are used in lighter vehicles like cars. Air brakes use compressed air to apply the service brakes and release the parking brake. When the driver presses the brake pedal, compressed air is released from the reservoir tanks through valves and hoses to the brake chambers, activating the brakes and causing the brake shoes or pads to engage with the drums or rotors, creating friction and slowing down the vehicle. Air brakes are particularly useful for large, heavy vehicles as they require a large force to stop and air brakes make this process easier for the driver.

Air Brake Systems vs Hydraulic Systems

Air brakes and hydraulic brakes are two very different braking systems that serve the same purpose: bringing a moving vehicle to a stop. While hydraulic brakes use fluid to generate stopping power, air brakes use compressed air. These systems are typically used on different types of vehicles, with hydraulic brakes being standard on passenger cars and light trucks, and air brakes being found on large trucks and buses.

Hydraulic Brakes

Hydraulic brakes use hydraulic fluid to transmit force when the brakes are applied. When the driver presses the brake pedal, the fluid pressure increases, forcing the brake pistons to push the brake pads against the drum or rotor, creating friction that slows down and eventually stops the vehicle. Hydraulic brakes are simple and effective, making them a common choice for smaller vehicles.

Air Brakes

On the other hand, air brakes rely on compressed air to operate. When the brake pedal is pressed, air fills the brake chamber, pushing a diaphragm that turns an "S-cam" and activates the brake pads. When the pedal is released, the air is released, allowing the brakes to disengage and the wheels to roll. Air brakes also have a unique safety feature: if there is a leak in the air brake lines, the air pressure decreases, activating the brakes and bringing the vehicle to a safe stop.

Advantages and Disadvantages

Hydraulic Brakes

One advantage of hydraulic brakes is their familiarity, as they operate similarly to brakes in passenger cars. They are also generally less expensive than air brakes, making them a cost-effective option for smaller vehicles. However, hydraulic brakes may not perform as well under heavy-duty or severe conditions, and they can be prone to reduced performance if pushed beyond their capacity. Additionally, if there is a leak in the hydraulic system, fluid pressure can drop, leading to a potential loss of braking power.

Air Brakes

Air brakes offer several advantages over hydraulic systems for larger vehicles. They have robust stopping power and are often considered safer due to their fail-safe mechanism. In the event of an air system failure or leak, the brakes are automatically applied, bringing the vehicle to a stop. Air brakes also have an unlimited supply of air, eliminating the risk of running out of brake fluid. Additionally, air brakes can be used for accessory applications, such as air horns and seat adjusters.

However, air brakes also come with some disadvantages. They are generally more expensive than hydraulic systems, and they require additional training for drivers to operate and maintain smoothly. Air brake systems can be more complex, and if the system fails or there is a split airline, the vehicle may become immobilized with the brakes stuck on. Air brakes also require proper maintenance, such as draining moisture from the system to prevent issues in cold locations.

Both air brake systems and hydraulic systems have their pros and cons. The choice between the two depends on various factors, including vehicle size, duty cycle, cost, and driver preference. While hydraulic brakes are suitable for lighter vehicles and offer a familiar and intuitive braking experience, air brakes are the preferred choice for larger trucks and buses due to their superior stopping power, safety features, and ability to handle heavy-duty applications.

Low-Pressure Warning Signals

Air brakes are a critical component of buses, providing reliable and efficient braking mechanisms. They use compressed air to control and facilitate the braking process. When a driver presses the brake pedal, compressed air is released from the reservoir tanks through valves and hoses to the brake chambers. This release of air pressure activates the brakes, causing the brake shoes or pads to engage with the drums or rotors, creating friction and slowing down the vehicle.

Modern air brake systems will warn the driver through both visual and audible signals when the system air pressure is critically low. This low-pressure warning system is an essential safety feature that helps prevent accidents and ensures the proper functioning of air brakes. When the system pressure drops to 60 psi (414 kPa) or greater, the low-air warning is activated. In some provinces, the warning must be activated above 55 psi (379 kPa). This warning alerts the driver that there is barely enough air pressure in the system to keep the spring parking brakes off.

If the system air pressure continues to drop, reaching a level below 45 psi (310 kPa), the emergency spring brakes will apply automatically. This automatic application of the emergency brakes is a critical safety feature to prevent accidents and ensure the vehicle can be brought to a stop even in the event of extremely low air pressure.

It is important to note that when the low-pressure warning activates, the driver must get the vehicle off the road immediately and investigate the source of the problem. The vehicle should not be operated until the issue is corrected, as driving with low air pressure can compromise the effectiveness of the brakes and pose a safety risk.

Overall, the low-pressure warning signals in air brake systems play a crucial role in maintaining the safety of buses and other vehicles that utilise air brakes. By alerting drivers to low air pressure, these warning signals help ensure that the braking system is functioning properly and can effectively slow down and stop the vehicle when needed.

Emergency Brakes

Air brakes are used in large, heavy vehicles such as buses, trucks, and trailers. They are a type of friction brake that uses compressed air to press on a piston, which in turn applies pressure to the brake pad to stop the vehicle. Air brakes are used as an alternative to hydraulic brakes, which are found in lighter vehicles such as cars.

The parking brake in an air brake system can also function as an emergency brake. This is because the parking brake is designed to be held in the 'applied' position by spring pressure. In other words, air pressure must be produced to release the parking brake. When the parking/emergency brake is set, the pressurized air in the lines between the compressed air storage tank and the brakes is released, allowing the spring-actuated parking brake to engage. Therefore, a sudden loss of air pressure would result in full spring brake pressure immediately, bringing the vehicle to a stop.

The parking brake can be activated by pulling a button on the dashboard. Before driving, the emergency brake button must be pushed in to fill the system with air. As long as the emergency system is pressurized, the emergency brake will remain free. However, if the system has a leak, the pressure can decrease enough to engage the emergency brake.

In addition to the parking brake, heavy trucks are often equipped with an exhaust brake that aids the braking process. This type of brake relies on the engine, rather than the air brake system.

Stop Light Switch

The Air Brake Service Circuit Stop Light Switch is an important component of a bus's air brake system. It is responsible for activating the brake lights when the driver applies the brakes, alerting other drivers that the bus is slowing down, stopping, or backing up. This switch is strategically placed under the dashboard and above the brake pedal.

When the driver presses the brake pedal, the lever arm attached to the pedal pushes a plunger inside the brake light switch, completing an electrical circuit. This action causes the rear brake lights to illuminate, providing a visual signal to other drivers. The switch is designed with silver contacts and a snap action mechanism to enhance its durability and ensure a long life span, undergoing testing at one million cycles at 30 AMPS.

The Air Brake Service Circuit Stop Light Switch is an essential safety feature that helps maintain orderly and safe traffic flow, especially when a bus is involved. By activating the brake lights, the switch communicates the bus's intention to slow down or stop, allowing other drivers to adjust their speed and maintain a safe distance. This simple yet effective mechanism contributes significantly to road safety and helps prevent rear-end collisions.

In addition to its role in the stop light circuit, the air brake system in buses serves a critical function in ensuring effective braking. Air brakes use compressed air to engage the brakes, providing the necessary force to stop these heavy vehicles. The system consists of several components, including air reservoirs, an air compressor, brake chambers, and brake shoes, all working in harmony to bring the bus to a safe stop when needed.

Overall, the Air Brake Service Circuit Stop Light Switch is an integral part of a bus's air brake system, serving the dual purpose of activating the brake lights and contributing to the overall functionality and safety of the braking mechanism. Through its design and strategic placement, this switch plays a vital role in ensuring a safe and smooth driving experience for both bus drivers and other motorists on the road.

Air Brake Noise

Air brakes use compressed air to control and facilitate the braking process in heavy vehicles such as buses. They are preferred over hydraulic brakes because air is more readily available than hydraulic brake fluid, which could leak and cause the brakes to fail.

Air brakes use compressed air to apply the service brakes and release the parking brake. When the driver presses the brake pedal, compressed air stored in the reservoir tanks is released through valves and hoses to the brake chambers. This release of air pressure activates the brakes, causing the brake shoes or pads to engage with the drums or rotors, creating friction and slowing down the vehicle.

Air brakes are known to make a squeaking or hissing sound when the air escapes after braking. This is caused by the automatic bypass safety valves opening when there is too much air pressure in the system. The "psss" sound is produced by these valves, which ensure that the air pressure remains at the correct level.

While air brake noise is generally not a cause for concern, it can be annoying or embarrassing for the driver, especially when it draws attention in traffic. In some cases, the noise may be due to an issue with the air release valve, and replacing it with a different type of valve can help reduce the noise.

It is important to maintain air brakes properly to ensure safe and efficient braking. This includes regularly checking the air pressure, draining water from the system, and addressing any air leaks. Poor maintenance of air brakes can lead to accidents.

Frequently asked questions